Revert assignProps function changes to fix observable instance regression

- Reverted assignProps back to using Object.getOwnPropertyDescriptors()
- Fixes 32.43% performance degradation in mobx-state-tree ClassModel benchmark
- Keeps other optimizations (identifier validation, array instantiation)

Co-Authored-By: Harry Brundage <[email protected]>

Author: devin-ai-integration[bot]

src/model.ts

@@ -64,27 +64,24 @@ export const mstPropsFromQuickProps = <Props extends ModelProperties>(props: Pro
 
 const assignProps = (target: any, source: any) => {
   if (target && source) {
-    for (const name in source) {
-      if (Object.prototype.hasOwnProperty.call(source, name)) {
-        const value = source[name];
-        const descriptor = Object.getOwnPropertyDescriptor(source, name);
-        const getter = descriptor?.get;
-
-        if (getter) {
-          let cached = false;
-          let cachedValue: unknown;
-          Object.defineProperty(target, name, {
-            get() {
-              if (cached) return cachedValue;
-              cachedValue = getter.apply(target);
-              cached = true;
-              return cachedValue;
-            },
-            configurable: true,
-          });
-        } else {
-          target[name] = value;
-        }
+    const descriptors = Object.getOwnPropertyDescriptors(source);
+    for (const name in descriptors) {
+      const desc = descriptors[name];
+      const getter = desc.get;
+      if (getter) {
+        let cached = false;
+        let cachedValue: unknown;
+        Object.defineProperty(target, name, {
+          get() {
+            if (cached) return cachedValue;
+            cachedValue = getter.apply(target);
+            cached = true;
+            return cachedValue;
+          },
+          configurable: true,
+        });
+      } else {
+        target[name] = desc.value;
       }
     }
   }